如何使用dplyr根据组选择数据框列的第一个值？

Question

I have 2 frames of data which I joined using the left_join() function. Then, I grouped the data by Group using the group_by() function. Using the mutate() function I want to create a column to repeatedly display the first value of column V2 according to the sort group.

In MWE the first value of V2 for Group 1 is 5 and for Group 2 it is 7.5. However, the code I wrote for this is selecting the first value from column V2 and repeating for both groups without separating as I want.

Note: it is simple because it seems to copy column V2 but this selection of the first value is necessary for me to do other calculations.

Any tips?

library(dplyr)

Group <- c(1, 2)
V1 <- c(10, 20, 30)
V2 <- c(5, 7.5)

df1 <- expand.grid(V1 = V1,
                   Group = Group) 

df2 <- data.frame(Group, V2)

df <- df1 %>%
  left_join(df2) %>%
  group_by(Group) %>%
  mutate(first = first(.$V2))

V1	Group	V2	first	The first column I want
10	1	5.0	5	5.0
20	1	5.0	5	5.0
30	1	5.0	5	5.0
10	2	7.5	5	7.5
20	2	7.5	5	7.5
30	2	7.5	5	7.5

Answer 1

Remove the .$ and it will work as .$ get the entire column breaking the group attribute and thus the first will be the first row value of the entire column

library(dplyr)
df1 %>%
  left_join(df2) %>%
  group_by(Group) %>%
  mutate(first = first(V2))